In LC-oscillators, tank-passives are chosen with respect to the operating frequency, the available power budget and features offered by the technology-metallization and transistor behavior. Phase noise in the oscillator results from noise generated in the oscillator passives and the active devices used to sustain oscillations.
Thermal noise generated by parasitic resistance present in the oscillator passives gets shaped by the frequency response of the tank. To minimize this noise, the selectivity of the filter may be increased. This coincides with the well-established notion of maximizing the tank Q to improve the phase noise. This claim is quantified in Leeson's equation, where phase noise power is seen to reduce with the square of the tank quality factor. While operating at the lower end of the gigahertz spectrum, tank Q is dominated by the quality factor of the inductor.
Series resistance is encountered due to the limited conductivity of metal traces; while losses to the substrate increasingly degrade inductor Q with frequency. To reduce the series resistance in monolithic inductors, one can increase both thickness and width of the metal trace. While interconnect thickness is a fixed feature offered by the technology being used, increasing the width of metal traces trades quality factor for self-inductance and frequency of self-oscillation. Use of a lower metal to electrically shield the passive from the substrate has been shown to reduce quality factor degradation. A floating fishbone structure has been used to shield the octagonal laid-out interleaved transformers. Magnetically induced eddy current losses in the substrate continue to exist. For a targeted frequency of oscillation in a given technology, and a fixed power budget, the tank can be optimized by trading width of traces, self-inductance and bandwidth appropriately. Availability of a thicker metal can provide a greater Q.
Conventionally, cross-coupled negative, i.e. −gm, oscillators usually form the basis for low-noise high-performance oscillator designs. Moreover, tail-current shaping, operation in class-C mode and higher order oscillators may also be deployed. However, the known oscillators may suffer from the fact that the noise contribution may convert to phase-noise. This may limit the lowest phase noise achievable.